Metol and phenidone are known allergens, and are cross-sensitizing. That means that someone that has developed a sensitivity towards metol will likely also be sensitive to phenidone and vice versa. For some, that rules out most of the PQ and MQ developers. I agree about the non-toxicity of caffenol, and its virtues as a developer, but I must note that it is very expensive to make. On the opposite end of the cost scale of home-made developers is parodinal (in use identical to Rodinal), and it too is not very toxic, especially when used in lower dilutions. It costs me about ZAR0,50 per film developed, that is about 5 EURO cents. Caffenol is at least ten times more expensive, if not more. And it smells funky, too . I think the phenidone-ascorbate developers deserve more attention, as they potentially cut the middle ground in terms of cost and environmental concerns. The quantity of phenidone used is miniscule, so should have very low impact in the bigger scheme of things.